Semiconductor integrated circuit device and microcomputer

ABSTRACT

A semiconductor integrated circuit comprises a logic circuit which is formed of p-channel MIS transistors and n-channel MIS transistors, a first oscillation circuit of variable oscillation frequency which is formed of p-channel MIS transistors and n-channel MIS transistors, a control circuit which produces a control signal for controlling the threshold voltage of the p-channel and n-channel MIS transistors, and a second oscillation circuit which produces multiple reference clock signals of different frequencies depending on the operation mode. The control circuit receives a reference clock signal and controls the first oscillation circuit with the control signal so that the oscillation frequency of the first oscillation circuit is correspondent to the frequency of the reference clock signal.

This is a Division of Ser. No. 08/622,389, filed Mar. 27, 1996.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor integrated circuit, andparticularly to a semiconductor integrated circuit device andmicrocomputer that are intended for fast and low-voltage operation, andto a microcomputer system based on this microcomputer.

In order for a MOS transistor to operate at a low voltage around 1 V, itmust have a lower threshold voltage for the enhancement of drivingability and operation speed. However, when the threshold voltage is settoo low, the MOS transistor cannot turn off completely due to itssubthreshold characteristics (tailing characteristics), causing asubthreshold leakage current to flow, resulting in an increased powerconsumption, as described in the "1993 Symposium on VLSI Circuits Digestof Technical Papers", pp. 45-46 (May 1993).

As the sub-micron MOS transistor pattern design advances, the variationamong devices of the fundamental characteristics including the thresholdvoltage attributable to the inequality of manufacturing processincreases, as described in the "1994 Symposium on VLSI Circuits Digestof Technical Papers", pp. 13-14 (June 1994).

FIG. 15 shows the variation of threshold voltage in connection with thegate length Lg of a MOS transistor. The variation of threshold voltagedue to the gate length variation increases as the gate length Lg becomesshorter.

Assuming the lower limit of threshold voltage to be 0.2 V for making thesubthreshold leakage current below a certain value and theabove-mentioned process causing the threshold variation to be ±0.15 V,the actual lower limit of threshold voltage, as the sum of these values,becomes 0.35 V.

On this account, conventional semiconductor integrated circuit devicescannot have their threshold voltage set much lower. Particularly, MOStransistors with lower power voltages operate in a state of incompletesaturation, and the operation speed of MOS transistor circuits fallssharply in response to a slight rise of the threshold voltage.Therefore, it is difficult for the conventional design methodology basedon the worst-case consideration to attain the intended performance ofsemiconductor integrated circuit devices.

SUMMARY OF THE INVENTION

The present invention is intended to overcome the foregoing prior artdeficiency.

An object of the invention is to provide a semiconductor integratedcircuit device formed of MOS transistors capable of properly arbitratingthe conflicting factors of the increased power consumption caused by thesubthreshold leakage current and the higher operation speed of MOStransistors.

Another object of the invention is to provide a semiconductor integratedcircuit device capable of controlling the threshold voltage properly sothat the power consumption and the operation speed are well balanced.

Still another object of the invention is to provide a semiconductorintegrated circuit device capable of controlling the threshold voltageeasily based on the external clock frequency.

Still another object of the invention is to provide a semiconductorintegrated circuit device which is operative at multiple clockfrequencies and capable of controlling the threshold voltage incorrespondence to a selected clock frequency.

Still another object of the invention is to provide a semiconductorintegrated circuit device capable of controlling the threshold voltageoptimally for each MOS transistor even if it is unequal among individualtransistors.

Still another object of the invention is to provide a microcomputerwhich is controlled to operate at an optimally balanced powerconsumption and operation speed, and a microcomputer system based on themicrocomputer.

In order to achieve the above objectives, the inventive semiconductorintegrated circuit device includes MOS transistors that constitute amain circuit and "delay-monitoring" MOS transistors that are provided incorrespondence to the main-circuit transistors. These MOS transistorsare operated under the corresponding substrate biases.

The operation speed or delay characteristics of the circuit is detectedbased on the comparison of a signal produced by the delay-monitoring MOStransistors with a clock signal or other reference timing signal. Thesubstrate bias level is controlled based on the result of detection sothat the main-circuit MOS transistors have an appropriate thresholdvoltage. The substrate bias level control is a sort of monitor controlbased on the delay-characteristic detecting transistors and thereference signal.

The difference of the delay characteristics of the circuit from thereference can be detected from a frequency error or phase error of thesignal produced by the delay-monitoring MOS transistors with respect tothe reference signal.

As a preferable arrangement, an oscillation circuit is designed suchthat the delay-monitoring MOS transistors determine the output frequencyof the oscillation circuit. In this case, the delay characteristics aredetected based on frequency error information that takes a continuoussignal form derived from the oscillation signal.

According to a preferred embodiment of this invention, a main circuitthat is a logic circuit is connected with a substrate-bias dependentoscillation circuit which shares the substrate bias with the maincircuit and an operation-mode dependent oscillation circuit whichswitches the output frequency depending on the operation mode, and asubstrate bias control circuit controls the substrate bias of the maincircuit by making both oscillation outputs synchronous.

A semiconductor integrated circuit device according to a preferredembodiment of this invention includes a logic circuit which implements acertain logical operation, a control circuit which controls thethreshold voltage of transistors that form the logic circuit, and avariable-frequency oscillation circuit, wherein the logic circuitincludes MOS transistors formed on a semiconductor substrate, theoscillation circuit has its output delivered to the control circuitwhich also receives a reference clock of a certain frequency and adaptedto produce a first control signal for controlling the oscillationcircuit so that the oscillation output has a frequency correspondent tothe frequency of the reference clock, and the MOS transistors of thelogic circuit have their threshold voltage controlled by a secondcontrol signal that corresponds to the first control signal.

A semiconductor integrated circuit device according to a preferredembodiment of this invention comprises a logic circuit including MIStransistors formed on a semiconductor substrate, a control circuit whichcontrols the threshold voltage of the MIS transistors of the logiccircuit, and an oscillation circuit including MIS transistors formed onthe semiconductor substrate and adapted to vary the output frequency,wherein the control circuit receives the output signal of theoscillation circuit and a clock signal of a certain frequency andcompares the oscillation output frequency with the clock signalfrequency to produce a first control signal, the oscillation circuit hasits threshold voltage controlled by the first control signal so that theoscillation output has a frequency corresponding to the clock signalfrequency, and the MIS transistors of the logic circuit have theirthreshold voltage controlled by a second control signal that correspondsto the first control signal.

A semiconductor integrated circuit device according to a preferredembodiment of this invention includes a logic circuit includingp-channel MIS transistors and n-channel MIS transistors, a firstoscillation circuit of variable frequency including p-channel MIStransistors and n-channel MIS transistors, a control circuit whichproduces a control signal for controlling the threshold voltages ofthese p-channel MIS transistors and n-channel MIS transistors, and asecond oscillation circuit which produces reference clocks of differentfrequencies depending on the operation mode, wherein the control circuitreceives one of the reference clocks and controls the first oscillationcircuit so that the oscillation output has a frequency that correspondsto the frequency of the reference clock.

A semiconductor integrated circuit device according to a preferredembodiment of this invention includes a logic circuit having at least afirst and second circuit blocks, a first and second oscillation circuitsof variable frequencies, a first control circuit which controls thethreshold voltage of MIS transistors of the first circuit block andfirst oscillation circuit, a second control circuit which controls thethreshold voltage of MIS transistors of the second circuit block andsecond oscillation circuit, a clock supply circuit which supplies aclock signal of a certain frequency to the first and second controlcircuits. The first control circuit controls the threshold voltage ofthe MIS transistors of the first circuit block and first oscillationcircuit so that the oscillation output frequency is equal to thefrequency of the clock signal and the second control circuit controlsthe threshold voltage of the MIS transistors of the second circuit blockand second oscillation circuit so that the oscillation output frequencyis equal to the frequency of the clock signal.

A microcomputer according to a preferred embodiment of this inventionincludes a load monitoring means for detecting the volume of processing(load) of the logic circuit formed of the above-mentioned semiconductorintegrated circuit device. The load monitoring means operates to varythe frequency of the clock signal depending on the detected volume ofprocessing.

According to the preferred embodiments of this invention, thesubstrate-bias dependent oscillation circuit provided in the circuitdevice operates at a known frequency selected for the operation mode ofthe logic circuit as the main circuit, whereby the threshold voltage ofMOS transistors of the oscillation circuit can be controlled optimallydepending on the operation mode.

Because of the substrate bias of the substrate-bias dependentoscillation circuit common to the substrate bias of the main circuit,the threshold voltage of MOS transistors used in the main circuit can becontrolled optimally in each operation mode. Consequently, it becomespossible to minimize the power consumption caused by the subthresholdleakage current. The oscillation circuit operates in synchronism with aknown frequency selected for each operation mode, allowing easyestimation of the delay of MOS transistors used in the main circuit andfacilitating the main circuit design.

According to the preferred embodiments of this invention, the thresholdvoltage of transistors can be controlled by a simple circuit arrangementbased on the reference clock, whereby the high operation speed and thesmaller power consumption can be balanced.

According to the preferred embodiments of this invention, theoscillation circuit has its output frequency controlled to the referenceclock frequency based on the threshold voltage control, whereby thelogic circuit of the semiconductor integrated circuit device can becontrolled efficiently.

According to the preferred embodiments of this invention, the logiccircuit of the circuit device has its threshold voltage varied to meeteach operation mode, whereby it has an optimal operation speed dependingon the operation mode.

According to the preferred embodiment of this invention, the logiccircuit is controlled by being divided into multiple blocks, whereby thethreshold voltage control can be implemented, with the inequality ofcharacteristics among transistors attributable to the manufacturingprocess being compensated.

According to the preferred embodiment of this invention, it is possibleto build a microcomputer in which the internal logic circuit has itsoperation speed controlled through the monitoring of the volume ofprocessing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the semiconductor integrated circuit basedon an embodiment of this invention;

FIG. 2 is a block diagram of the semiconductor integrated circuit basedon another embodiment of this invention;

FIG. 3 is a block diagram of the semiconductor integrated circuit basedon still another embodiment of this invention;

FIG. 4 is a block diagram showing more specific circuit arrangement ofthe embodiment shown in FIG. 3;

FIGS. 5A and 5B are schematic diagrams each showing the detailed circuitarrangement of the frequency/phase comparator PFD1 and low-pass filterLPF1 used in the circuit shown in FIG. 4;

FIG. 6 is a timing chart showing the relation of the operation frequencyand the substrate bias of the embodiment shown in FIG. 4;

FIGS. 7A and 7B are graphs showing the relation of the substrate biasand the threshold voltage of an NMOS transistor and PMOS transistor,respectively;

FIG. 8 is a graph showing the relation of the operation frequency andthe substrate bias of a MOS transistor;

FIG. 9 is a graph showing the relation of the gate voltage and the draincurrent of a MOS transistor;

FIG. 10 is a graph showing the relation of the operation frequency andthe power consumption based on an embodiment of this invention;

FIG. 11 is a cross-sectional diagram showing an example of the devicestructure of the semiconductor integrated circuit based on an embodimentof this invention:

FIG. 12 is a block diagram of the semiconductor integrated circuit basedon still another embodiment of this invention;

FIG. 13 is a graph showing the relation of the gate voltage and thedrain current of the inventive MOS transistor;

FIG. 14 is a block diagram of the semiconductor integrated circuit basedon still another embodiment of this invention:

FIG. 15 is a graph showing the relation of the gate length and thethreshold voltage of the inventive MOS transistor;

FIG. 16 is a block diagram of the microcomputer based on an embodimentof this invention; and

FIG. 17 is a block diagram of the microcomputer system based on anembodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of this invention will be explained in detail withreference to the drawings.

FIG. 1 shows the first embodiment of this invention, in which thesemiconductor integrated circuit device includes a main circuit LOGOsuch as a logic circuit, a variable-frequency oscillation circuit OSC0,and a control circuit CNT0.

The oscillation circuit OSC0 receives a control signal B1 from thecontrol circuit CNT0 and has its oscillation frequency dependent on thevalue of the control signal B1. The control circuit CNT0 receives anexternal clock signal CLK0 and the oscillation output S0 of theoscillation circuit OSC0, and produces control signals B0 and B1 thatare proportional to the difference of frequency between the externalclock signal CLK0 and oscillation output S0. The control signal B0 isfed to the main circuit LOG0, while the control signal B1 is fed to theoscillation circuit OSC0.

The control circuit CNT0 and variable-frequency oscillation circuit OSC0in unison form a stable negative feedback system so that the frequencyof the oscillation output S0 of the oscillation circuit OSC0 correspondsto the frequency of the external clock signal CLK0. For example, thesefrequencies are equal, and the frequency of S0 is locked to thefrequency of CLK0, as will be explained in more detail later.

The oscillation circuit OSC0 is formed of n-channel MOSFETs andp-channel MOSFETs formed on a semiconductor substrate, with thesubstrate bias being varied by the control voltage provided by thecontrol circuit CNT0. The variation of the substrate bias varies thethreshold voltage, and thus varies the oscillation frequency.

The main circuit LOG0 receives the control signal B0 from the controlcircuit CNT0, and its MOSFETs have their substrate bias and thus theirthreshold voltage controlled by the signal B0. This circuit arrangementenables to control the threshold voltage of the main circuit andaccordingly vary its power consumption and operation speed in responseto the external clock frequency.

More specifically, the oscillation circuit OSC0 receives the controlsignal Bi from the control circuit CNT0 and oscillates at a certainfrequency. The oscillation circuit OSC0 is controlled by the controlsignal B1 so that the frequency of oscillation output S0 corresponds tothe frequency of external clock signal CLK0. The control signal B1controls the substrate bias of MOS transistors of the oscillationcircuit OSC0 to thereby control their threshold voltage, and the MOStransistors operate at such an operation speed as to oscillate at thefrequency that corresponds to the frequency of external clock signalCLK0.

Accordingly, by controlling the substrate bias of the main circuit LOG0with the signal B0 which corresponds to the control signal B1 for thesubstrate bias of the oscillation circuit OSC0, it is possible tocontrol the threshold voltage of the MOS transistors of the main circuitLOG0 so that it operates at the operation speed which corresponds to theexternal clock signal CLK0.

The control circuit CNT0 consists of a frequency/phase comparator (notshown in detail) which detects the difference of frequency between theexternal clock signal CLK0 and the oscillation output S0, and a controlsignal generation circuit which produces in response to the output ofthe frequency/phase comparator a control signal B0 to be fed to the maincircuit LOG0 and a control signal B1 to be fed to the gates of MOSFETsof the oscillation circuit OSC0. The frequency/phase comparator, whichhas a delay detecting function or operation speed detecting function,delivers frequency error information in terms of the delay or operationspeed of the main circuit LOG0 and oscillation circuit OSC0. In thisinvention, the frequency and phase have the same significance inrepresenting the delay or operation speed, although these items shouldbe treated separately in concept since phase control accompaniesfrequency control as will be appreciated in the following explanation.

The control signals B0 and B1 have certain reference levels when theoscillation output S0 corresponds in frequency to the external clocksignal CLK0, or vary to increase the substrate bias level if thefrequency of S0 is higher than CLK0 or decrease the substrate bias levelif the frequency of S0 is lower than CLK0.

It is known that a larger substrate bias raises the threshold voltagelevel of the related MOSFETs and a smaller substrate bias lowers theirthreshold voltage level. Accordingly, if the control signal B1 increasesthe substrate bias, the MOSFETs of oscillation circuit OSC0 have theirconductance decreased accordingly, causing the time constant of OSC0,which is determined from the MOSFETs and parasitic capacitance, toincrease, resulting in a lower oscillation frequency. Whereas, adecreased substrate bias causes the threshold voltage of MOSFETs to falland their conductance to increase, resulting in a higher oscillationfrequency of the oscillation circuit OSC0. In this manner, theoscillation frequency is controlled through the control of the thresholdvoltage of MOSFETs.

In the embodiment of FIG. 1, the control signal generation circuit isdesigned to produce two control signals B0 and B1, which facilitates thecircuit design and the achievement of intended response characteristics.In case the main circuit LOG0 and oscillation circuit OSC0 need to besupplied with control signals for their individual substrate biasing orin case it is intended to reduce the electrical coupling between themain circuit LOG0 and oscillation circuit OSC0 through their substrate,the control signal generation circuit may be two separate circuits forproducing the control signals B0 and B1. In this case, the two controlsignal generation circuits receive the output of the frequency/phasecomparator to produce control signals B0 and B1 having an equal signallevel. The two control signal generation circuits may have differentcircuit arrangements.

For example, in case the main circuit LOG0 has two states of active modeand sleep mode, the control signal B0 at sleep mode specifies a highersubstrate bias in response to a certain sleep-mode command signalirrespective of the detected level of the frequency/phase comparator.With the control signal B1 being made responsive to the output of thefrequency/phase comparator, the oscillation circuit OSC0 is controlledby the signal. Consequently, the power consumption can be reduced duringsleep mode of the main circuit LOG0, and the control signal B0 to themain circuit LOG0 can be restored quickly when the circuit LOG0 hasregained active mode.

Generally, when a high oscillation frequency is selected for thehigh-speed operation by setting a lower threshold voltage, the powerconsumption increases due to the subthreshold leakage current, whereaswhen a lower oscillation frequency is selected by setting a higherthreshold voltage, the power consumption decreases in exchange for alower operation speed. When the main circuit LOG0 is a logic circuitthat bases the operation on an external clock signal CLK0, the circuitoperates at the speed corresponding to the external clock CLK0 throughthe control of the threshold voltage of MOS transistors of the maincircuit LOG0. The threshold voltage is controlled so that the powerconsumption is smaller when the operation speed is lower.

Although in this embodiment the frequency of oscillation output S0 iscontrolled through the control of substrate bias of the oscillationcircuit OSC0, another control scheme may be adopted by making theoperation speed of MOS transistors variable in correspondence to thefrequency of external clock signal CLK0. In this case, the oscillationcircuit OSC0 functions as a means of monitoring the threshold voltage ofthe main circuit LOG0 for achieving the operation speed that correspondsto the frequency of external clock signal CLK0, and therefore it isnecessary to convert the signal B1 for controlling the oscillationfrequency of oscillation circuit OSC0 into the signal B0 for controllingthe threshold voltage of the main circuit LOG0.

Although the foregoing embodiment is designed to vary the substrate biasin order to control the threshold voltage of transistors used in themain circuit LOG0, the transistor threshold voltage may be controlled ina different manner instead of the substrate bias control.

In contrast to the conventional variable-frequency oscillation circuit,in which in general the operation speed of the inverter circuit whichconstitutes a ring oscillator is made variable through the additionalprovision of transistors for controlling the supply current to theinverter circuit, the circuit arrangement for varying the oscillationfrequency through the variation of substrate bias as in this embodimenteliminates the need of these current control transistors and suppliesthe power voltage to the inverter circuit without a loss, raises theupper limit of oscillation frequency, and expands the range of variablefrequency.

The circuit arrangement of this embodiment also enables the oscillationfrequency of the oscillation circuit to be locked to the frequency of anexternal clock signal. This makes it possible to build a phase-lockedloop (PLL) circuit which can follow a high-frequency external clocksignal.

FIG. 2 shows another embodiment of this invention. The principal portionof this embodiment is identical to the first embodiment, and detailedexplanation thereof is omitted. This embodiment differs from the firstembodiment shown in FIG. 1 in the use of a common control signal B3 inplace of the control signals B0 and B1 for controlling the thresholdvoltage and oscillation frequency. Accordingly, the oscillationfrequency of the oscillation circuit OSC0 is controlled through thevariation of the substrate bias by the control signal B3 provided by thecontrol circuit CNT0, and the substrate bias of the main circuit LOG0 isalso controlled by the same control signal B3.

This embodiment using the common control signal B3 of the controlcircuit CNT0 simplifies the circuit arrangement and yet accomplishes thesame effectiveness as the first embodiment. The embodiment enables tobuild an oscillation circuit having a wide range of frequency and a PLLcircuit which can follow a high-frequency external clock signal. It alsoenables to control the operation speed and power consumption of the maincircuit in response to the external clock frequency.

FIG. 3 shows still another embodiment of this invention, in whichsubstrate bias of MOS transistors is used for the threshold voltagecontrol.

The semiconductor integrated circuit device includes a main circuit LOG1and a substrate-bias dependent oscillation circuit OSC1 which varies theoscillation frequency in response to the substrate bias. A substratebias control circuit CNT1 uses the output S1 of the oscillation circuitOSC1 and the oscillation output CLK1 of an operation-mode dependentoscillation circuit VCLK1 to produce substrate voltages BP1 and BN1 forthe main circuit LOG1.

The oscillation circuit OSC1 of this embodiment is a ring oscillatormade up of series-connected inverters formed of p-channel MOSFETs andn-channel MOSFETs on a semiconductor substrate. The main circuit LOG1 isa CMOS logic circuit formed of p-channel MOSFETs and n-channel MOSFETs.The substrate bias voltages BP1 and BN1 are applied to the n-substrate(or n-well electrode) of PMOS transistors and to the p-substrate (orp-well electrode) of NMOS transistors, respectively, of the main circuitLOG1.

The operation-mode dependent oscillation circuit VCLK1 which operatesbased on the system clock SCLK1 as the reference clock to produce anoscillation output CLK1 having a frequency that depends on an internalor external operation mode signal MODE1. The main circuit LOG1 is alogic circuit which operates based on the oscillation output CLK1 as theclock in this embodiment.

The substrate bias voltages BP1 and BN1 are also supplied to thesubstrate-bias dependent oscillation circuit OSC1. The substrate-biasdependent oscillation circuit OSC1 and the substrate bias controlcircuit CNT1 which receives the output S1 of the oscillation circuitOSC1 as a feedback signal form a stable negative feedback system so thatthe output S1 of the oscillation circuit OSC1 has its frequency lockedto the frequency of oscillation output CLK1 of the operation-modedependent oscillation circuit VCLK1.

The operation mode mentioned here is set in advance based on theintended operation speed and power consumption, such as high-speedoperation mode, low-speed operation mode and sleep mode, of the maincircuit LOG1 in this embodiment.

Since both substrate bias voltages of the substrate-bias dependentoscillation circuit OSC1 and main circuit LOG1 are controlled by theoutputs BP1 and BN1 of the substrate bias control circuit CNT1, it ispossible to control the substrate bias of the main circuit LOG1 by theoscillation output CLK1 of the operation-mode dependent oscillationcircuit VCLK1. The MOS transistors have their threshold voltage variedby the substrate bias voltage, and accordingly it is possible to switchthe threshold voltage of MOS transistors of the main circuit LOG1appropriately to meet the operation mode by switching the operation modesignal MODE1.

Since the main circuit LOG1 has its operation speed and powerconsumption varied in response to the threshold voltage as mentionedabove, it is possible to control the operation speed and powerconsumption of the main circuit LOG1 appropriately to meet the operationmode by switching the frequency of the clock signal CLK1 supplied to thecontrol circuit CNT1 by use of the operation mode signal MODE1.

Although in this embodiment and following embodiments, the oscillationfrequency of the operation-mode dependent oscillation circuit VCLK1 isvaried with the signal indicative of the operation mode thereby toproduce the oscillation output CLK1, an alternative arrangement is tofeed an external clock signal to the control circuit CNT1 directly, asshown in FIG. 1 and FIG. 2. In this case, it becomes possible to controlthe operation speed and power consumption of the internal main circuitLOG1 through the prior determination of the frequency of the externalclock (system clock), and the circuit arrangement can be simplifiedowing to the elimination of the operation-mode dependent oscillationcircuit VCLK1.

Although in this embodiment the substrate bias voltages of both ofn-type MOSFETs and p-type MOSFETs are controlled with the BP1 and BN1voltages, an alternative arrangement is to use one of BP1 and BN1 tovary the substrate bias of MOSFETs of only one type.

More specific circuit arrangements of the foregoing embodiment will beexplained in detail in connection with FIG. 4, FIGS. 5A and 5B, and FIG.6.

FIG. 4 shows in detail the internal circuit arrangement of each circuitblock shown in FIG. 3. The substrate-bias dependent oscillation circuitOSC1 is a ring oscillator made up of 5-stage CMOS inverter circuits(each p-channel or n-channel MOSFET has a gate length of 0.25 μm and agate width of 5 μm). The MOS transistors of the ring oscillator havetheir substrate electrodes (or well electrodes) used for the oscillationfrequency control line.

The main circuit LOG1 is a CMOS logic circuit (gate length: 0.25 μm;gate width: 5 μm) of 2-input NAND gate having input signals A and B andan output signal C as an example of this embodiment. The substrate biascontrol circuit CNT1 consists of a substrate bias generation circuitBGEN1, a low-pass filter LPF1 and a frequency/phase comparator PFD1.FIG. 5A shows the specific circuit arrangement of the frequency/phasecomparator PFD1, and FIG. 5B shows the specific circuit arrangement ofthe low-pass filter LPF1.

The frequency/phase comparator PFD1, which has a well-known circuitarrangement, receives a reference frequency signal REF1 and a signal S1of a certain frequency and produces an output signal S2 indicative ofthe phase difference of the two input signals. The low-pass filter LPF1is a well-known lag-lead filter formed of resistors and a capacitor.Further explanation of these circuit blocks are omitted.

The substrate bias generation circuit BGEN1 is a DC-to-DC converter forconverting the output voltage S3 of the low-pass filter LPF1 into anoptimal voltage level for the substrate bias. The control signal BP1delivered by the substrate bias generation circuit BGEN1 is higher thanthe power voltage Vdd, while the control signal BN1 is lower than theground voltage Vss.

The control signals BP1 and BN1 and the output voltage S3 of thelow-pass filter LPF1 have the following relationship. If the frequencyof output S1 of the oscillation circuit OSC1 is lower than the frequencyof the clock signal CLK1 as the reference signal REF1, the low-passfilter LPF1 shown in FIG. 5B responds to the output S2 of thefrequency/phase comparator PFD1 shown in FIG. 5A to produce a voltage(detected signal) S3 having a relatively lower voltage level close tothe ground voltage Vss. In response to the lower detected signal S3, thecontrol signal BP1 has a relatively lower voltage level close to thepower voltage Vdd, and the control signal BN1 has a relatively lowervoltage level close to the ground voltage Vss.

Otherwise, if the frequency of the oscillation output S1 is higher thanthe frequency of the clock signal CLK1, the low-pass filter LPF1delivers a detected signal S3 having a relatively higher voltage levelclose to the power voltage Vdd. In response to the higher detectedsignal S3, the control signal BP1 has a relatively higher voltage levelabove the power voltage Vdd and the control signal BN1 has a deeply lowvoltage level below the ground voltage Vss.

The control signals BP1 and BN1 have voltage levels that are beyond theoperational voltage range Vdd-Vss of the circuit and have acomplementary transition of voltage levels in response to the detectedsignal S3. Through the control of conductance of MOSFETs based on thecontrol of substrate bias as described above, the oscillation circuitOSC1 and main circuit LOG1 operate as intended.

The substrate bias generation circuit BGEN1 which produces these controlsignals BP1 and BN1 can be configured in a relatively simple circuitarrangement including a substrate bias oscillation circuit, voltagelevel control circuit, positive charge pump circuit, and negative chargepump circuit, for example. In this case, the voltage level controlcircuit controls the level of the oscillation signal fed to the positivecharge pump circuit and negative charge pump circuit in accordance withthe output S3 of the low-pass filter LPF1.

The voltage level control circuit, which receives the output S3 of thelow-pass filter LPF1, is configured as a voltage follower circuit formedof a MOS operational amplifier, for example. The output of the voltagefollower circuit is considered to be an internal voltage source, and itis used at least for the power source of the oscillation signal outputcircuit of the substrate bias oscillation circuit. Based on this circuitarrangement, the oscillation signal output circuit has its output signallevel determined by that power voltage, and it is controlled to have avoltage level which corresponds to the voltage level of the output S3 ofthe low-pass filter LPF1.

The positive charge pump circuit is made up of a positive charge pumpcapacitor which consists of a MOS capacitor having its one terminalcoupled to the output terminal of the oscillation signal output circuit,a first rectifying element which consists of a p-channel MOSFET in diodeconfiguration having its source connected to the power terminal Vdd andits gate and drain connected to another terminal of the positive chargepump capacitor, and a second rectifying element which consists of ap-channel MOSFET in diode configuration having its source connected tothe other terminal of the capacitor and its gate and drain connected tothe substrate gates of the p-channel MOSFETs of the main circuit LOG1and oscillation circuit OSC1. The positive charge pump circuit providesperiodically a positive charge to the substrate gate of the p-channelMOSFETs in response to the oscillation signal from the substrate biasoscillation circuit. This positive charge is smoothed by a parasiticcapacitance such as a junction capacitance existing in the substrategate, and it provides the substrate gate with a bias voltage as thecontrol signal BP1.

The negative charge pump circuit is made up of an negative charge pumpcapacitor which consists of a MOS capacitor having its one terminalcoupled to the output terminal of the oscillation signal output circuit,a third rectifying element which consists of an n-channel MOSFET indiode configuration having its source connected to the ground terminalVss and its gate and drain connected to another terminal of the negativecharge pump capacitor, and a fourth rectifying element which consists ofan n-channel MOSFET in diode configuration having its source connectedto substrate gates of the n-channel MOSFETs of the main circuit LOG1 andoscillation circuit OSC1 and its gate and drain connected to the otherterminal of the negative charge pump capacitor.

In case another voltage source such as an external power source havingan output voltage level higher than the power voltage Vdd is available,the circuit for producing the control signal BP1 in the substrate biasgeneration circuit BGEN1 may be arranged as follows. A level conversioncircuit or level shift circuit which is supplied with power by theabove-mentioned another voltage source is provided, by which the controlsignal BP1 is produced from the output of the low-pass filter LPF1through the conversion or level shift. Similarly, if another voltagesource such as an external power source having an output voltage levellower than the ground voltage Vss is available, the control signal BN1may be produced by use of a similar level conversion circuit or levelshift circuit.

By using the level conversion circuit or level shift circuit, it becomespossible to Reduce the number of relatively large circuit componentssuch as the charge pump capacitors, and also reduce the transitionalcurrent of charge pump operation which can create noises in the powerwiring system.

In FIG. 4, the operation-mode dependent oscillation circuit VCLK1 ismade up of a frequency multiplier PLL1 and operation mode controlcircuit MCU1. A fixed-frequency oscillator CK1 is used for the referenceclock source of the operation-mode dependent oscillation circuit VCLK1.

The frequency multiplier PLL1 has its multiplying factor varied by theoperation mode control circuit MCU1 in accordance with a mode controlsignal MODE associated with the operation of the main circuit LOG1. Theoutput frequency of the fixed-frequency oscillator CK1 is multiplied bya multiplying factor that depends on the operation mode of the maincircuit LOG1, and a resulting oscillation output CLK1 with a rectangularwaveform and a frequency f0 (ranging from 0 to 100 MHz) is released fromthe frequency multiplier PLL1.

The oscillation output S1 of the CMOS ring oscillator OSC1 is fedtogether with the oscillation output CLK1 of the frequency multiplierPLL1 to the frequency/phase comparator PFD1, which has its output S2 fedthrough the low-pass filter LPF1 to the substrate bias generationcircuit BGEN1, by which the substrate bias voltages BP1 and BN1 areproduced.

The substrate bias voltages BP1 and BN1 are applied commonly to the CMOSring oscillator OSC1 and main circuit LOG1. The substrate bias voltageBN1 is applied commonly to the substrate electrodes of n-type MOSFETs(transistors with gate-pointing arrows at the channel section), whilethe substrate bias voltage BP1 is applied commonly to the substrateelectrodes of p-type MOSFETs (transistors with gate-originating arrowsat the channel section), as shown in FIG. 4. These transistors havetheir operating point set so as to operate in a low voltage range ofabout 1 V based on a power voltage Vdd of about 1 V and a ground voltageVss.

FIGS. 7A and 7B show the relationship of the substrate bias voltage andthreshold voltage of the MOS transistors. FIG. 7A is for the NMOStransistors, and FIG. 7B is for the PMOS transistors.

The MOS transistors have their threshold voltage varied depending on thesubstrate bias as shown in FIGS. 7A and 7B and have a reduced currentdrive ability for a smaller value of threshold voltage in general, andaccordingly the CMOS ring oscillator OSC1 of FIG. 4 operates as avoltage-controlled oscillator (VCO) in response to the substrate biassignals BP1 and BN1. On this account, the whole circuit of FIG. 4 has aphase-locked loop (PLL) structure and operates so that the oscillationfrequency and phase of the CMOS ring oscillator OSC1 coincide with thoseof the oscillation output CLK1 of the frequency multiplier PLL1.

The oscillaion circuit (VCO) and PLL circuit of this embodiment enableto build an oscillation circuit having a wide frequency range and a PLLcircuit capable of following an external high-frequency clock signal,and also enable the control of operation speed and power consumption ofthe main circuit depending on the external clock frequency. The PLLcircuit is described in publication "IEEE JOURNAL OF SOLID-STATECIRCUITS", Vol. 22, No. 2 (April 1987), pp. 255-261, for example.

Next, the variation of the substrate bias voltages BP1 and BN1 andthreshold voltage of the MOS transistors of the main circuit LOG1 inresponse to the switching of operation mode of the main circuit LOG1will be explained in connection with the timing chart of FIG. 6.

When the operation mode is switched, the operation mode control circuitMCU1 switches the frequency multiplying factor of the frequencymultiplier PLL1, and its oscillation output CLK1 varies. In thisexample, the frequency f0 varies with time from 75 MHz to 50 MHz.Consequently, the substrate bias voltage BP1 rises from 1.3 V to about1.6 V, and the substrate bias voltage BN1 falls from -0.3 V to about-0.6 V. Consequently, the threshold voltage of MOS transistors rises(refer to FIGS. 7A and 7B), causing the oscillation frequency f1 of thesubstrate-bias dependent oscillation circuit OSC1 to fall gradually tobecome equal to the frequency f0 (50 MHz) of oscillation output CLK1 ofthe frequency multiplier PLL1. At the same time, the output of thecircuit OSC1 becomes in-phase with the oscillation output CLK1 of thefrequency multiplier PLL1.

If the frequency f0 of the oscillation output CLK1 of the frequencymultiplier PLL1 rises from 50 MHz to 100 MHz in response to theswitching of the operation mode signal MODE, the substrate bias voltageBP1 falls from 1.6 V to about 1.0 V and the substrate bias voltage BN1rises from -0.6 V to about 0 V. Consequently, the threshold voltage ofMOS transistors falls (refer to FIGS. 7A and 7B), causing theoscillation frequency f1 of the substrate-bias dependent oscillationcircuit OSC1 to rise gradually to become equal to the frequency f0 (100MHz) of oscillation output CLK1 of the frequency multiplier PLL1.

FIG. 8 shows the relationship of the frequency f0 of oscillation outputCLK1 and the substrate bias voltages BP1 and BN1 of the frequencymultiplier PLL1. In this manner, the substrate bias of the MOStransistors of the substrate-bias dependent oscillation circuit OSC1 canbe varied depending on the oscillation output CLK1 of the frequencymultiplier PLL1, and the threshold voltage of the MOS transistors can bevaried accordingly.

Although the frequency f0 of oscillation output CLK1 is switched between50 MHz, 75 MHz and 100 MHz in this embodiment, arbitrary oscillationfrequencies that meet the control range of threshold voltage can easilybe chosen. It is possible for this embodiment to set the oscillationfrequency f1 of the CMOS ring oscillator OSC1 to a preset value,enabling the estimation of the signal propagation delay time of MOStransistors of the main circuit LOG1 in each operation mode.

FIG. 9 shows the gate voltage dependency of MOS transistors in theirsubthreshold region of the drain current. Generally, the lower thethreshold voltage of MOS transistors, the greater is their drivingability, enabling higher-speed operation. However, the comparison ofpoints A and B in FIG. 9 reveals that the on-off ratio of MOStransistors decreases, the subthreshold current increases, and the powerconsumption increases.

According to the inventive circuit arrangement, it is possible forhigh-speed operation to lower the threshold voltage of MOS transistorsof the main circuit LOG1 by choosing from outside an operation mode fora higher oscillation frequency of the output CLK1 of frequencymultiplier PLL1. Although in this case the subthreshold current andpower consumption of main circuit LOG1 increase, it can operate at ahigher speed. It is also possible for low-speed operation to raise thethreshold voltage of MOS transistors of the main circuit LOG1 bychoosing from outside an operation mode for a lower oscillationfrequency of the output CLK1 of frequency multiplier PLL1, therebyreducing the subthreshold current and power consumption of the maincircuit LOG1.

FIG. 10 shows the relation between operation frequency and powerconsumption. Curve P2 shows the power consumption of the main circuitLOG1 when the operation mode signal MODE is varied to change thefrequency f0 of oscillation output CLK1 of the frequency multiplierPLL1. The main circuit LOG1 operates at the same frequency f0 of theoscillation output CLK1 of PLL1 in this example. Line P0 shows the powerconsumption of LOG1 without power loss in subthreshold current T0, andline P1 shows the power consumption with the subthreshold current T0.

The power consumption of the main circuit LOG1 has a linear relationshipwith its operation frequency f0 as shown. The subthreshold current doesnot depend on the operation frequency f0, and line P1 is parallel toline P0. In the case of line P2 relevant to this invention, thethreshold voltage which relates to the subthreshold current varies withthe operation frequency f0, and the power consumption approaches the P0without power loss in subthreshold current as the operation frequency f0falls in response to the operation mode signal. Consequently, the maincircuit LOG1 can be operated at a minimal power consumption caused by aminimal subthreshold current needed for the operation at the operationfrequency f0. This effect is particularly significant for thelow-voltage operation around 1 V in which case the subthreshold currentis material and for higher-density circuit integration.

Since the threshold voltage of the main circuit LOG1 is controlledautomatically to take the preset value depending on the operation modeof the main circuit LOG1, the inventive circuit arrangement can copewith unequal characteristics among MOS transistors of the main circuitLOG1 and external varying factors such as the temperature and powervoltage.

Although FIG. 4 shows a 2-input NAND gate as an example of the maincircuit LOG1 of FIG. 3, it can be replaced with an inverter or otherlogic gates such as a NOR gate, or can be replaced with a logic circuitmade up of these logic gates. The main circuit LOG1 of CMOS transistorscan alternatively be formed of NMOS transistors and/or PMOS transistors,or can include bipolar transistors. The CMOS ring oscillator shown inFIG. 4 can also be replaced with any other oscillation circuit thatvaries the output frequency in response to the substrate bias.

FIG. 11 shows the cross section of the CMOS structure for fabricatingthe integrated circuit shown in FIG. 4.

N well 109 and p well 110 are formed on part of a p-type siliconsubstrate 111. Formed on the surface of the n well 109 is a PMOStransistor consisting of a p+ type source, drain diffusion regions 103and 104, a gate electrode 107 and a gate oxide film 112, and formed onthe surface of the p well 110 is an NMOS transistor consisting of a n+type source, drain diffusion regions 105 and 106, a gate electrode 108and a gate oxide film 113. Device isolation films 100,101 and 102 areformed between the PMOS transistor and NMOS transistor. Power lines forsupplying the substrate bias voltages BP1 and BN1 to the PMOS transistorand NMOS transistor are connected to the respective well regions,although these lines are not shown in the figure.

An n-type substrate may be used instead of the p-type substrate of thisexample. Instead of the twin well structure using the n well 109 and pwell 110 in the example of FIG. 11, a single well structure, with one ofthe n well and p well being substituted by the substrate, may be used.

MOS transistors of the triple well structure as described in publication"ISSCC Digest of Technical Papers" (February 1989), pp. 248-249, or MOStransistors of the S0I structure as described in publication "1992 IEDMTechnical Digest", pp. 35-38 may be formed.

Although in the foregoing embodiments, the substrate bias voltage forNMOS transistors is 0 V or lower and that of PMOS transistors is thepower voltage (e.g., 1.0 V) of the main circuit or higher, it may beapplied so as to provide a forward bias for the pn junction between thediffusion layer of MOS transistors and the substrate.

Particularly, if the forward bias voltage does not exceed the diffusionpotential (around 0.6 V), the leakage current between the diffusionlayer and substrate is small enough to prevent an increased powerconsumption, enabling the application of a forward bias. In this case,the substrate bias factor of the threshold voltage (the variation ofthreshold voltage against the substrate bias) is large in the substratebias region, and therefore the threshold voltage of MOS transistors canbe controlled efficiently. Another advantage is the elimination of theneed of a special substrate biasing circuit such as a negative voltagesource owing to the setting of the substrate bias voltage within therange of power voltage.

Although in the foregoing embodiments, the threshold voltage of MOStransistors of the main circuit is controlled through the control ofsubstrate bias, an alternative scheme is to configure a main circuitwith MOS transistors having a threshold voltage control terminal (e.g.,SOIMOSFETs having an electrode that is insulated from the siliconsubstrate) and apply a substrate bias control voltage to the terminal.

FIG. 12 shows an embodiment of the configuration of a main circuit LOG1that is divided into circuit blocks LOG10, LOG20 and LOG30. The circuitblocks LOG10-LOG30 have their substrate bias voltages BP10-BP30 andBN10-BN30 controlled by the oscillation output CLK10 of a singleoperation-mode dependent oscillation circuit VCLK10. The oscillationoutput CLK10 of the circuit VCLK10 is fed commonly to the controlcircuits CNT10-CNT30 that are provided along with oscillation circuitsOSC10-OSC30 for the respective circuit blocks LOG10-LOG30. Theoperation-mode dependent oscillation circuit VCLK10 and each of thecontrol circuits CNT10-CNT30 and oscillation circuits OSC10-OSC30 arearranged as shown in FIG. 4.

In this embodiment, the substrate bias of the circuit blocks LOG10-LOG30are controlled independently by the oscillation output CLK10 of theoperation-mode dependent oscillation circuit VCLK10. Accordingly, it ispossible to compensate the inequality of threshold voltage and substratebias characteristics of MOS transistors among the circuit blocksLOG10-LOG30. If MOS transistors of the main circuit LOG10 have athreshold voltage different from that of other circuits due to themanufacturing process, MOS transistors of the oscillation circuit OSC10conceivably have the same variation of threshold voltage, and thesubstrate bias for these MOS transistors can be controlled appropriatelyby the clock signal CLK10. Specifically, if the threshold voltagediffers by 0.15 V among the circuit blocks LOG10-LOG30, the differencecan be reduced to around 0.05 V according to this embodiment.

Since the inequality of circuit characteristics attributable to themanufacturing process is dependent on the location on the semiconductorchip of the semiconductor integrated circuit, it is desirable to arrangeeach main circuit LOG and associated oscillation circuit OSC close toeach other. Similarly, when dividing a main circuit into circuit blocks,it is desirable to partition a semiconductor chip symmetrically intotwo, four, or the like so that a circuit block is configured withtransistors that are close to each other.

The allowable range of threshold voltage has its lower limit determinedfrom the subthreshold leakage current and its upper limit determinedfrom the required operation speed, as mentioned previously. On thisaccount, it is obliged conventionally to set a higher threshold voltageagainst high-speed circuit operation when the threshold voltage differgreatly among transistors, whereas the scheme of this embodiment allowsthe setting of threshold voltage to its lower limit, enabling thehigh-speed circuit operation.

The effectiveness of these embodiments is particularly significant forthe low-voltage operation at a power voltage of around 1 V.

FIG. 13 shows the variation of drain current when the threshold voltagevaries with the gate voltage applied to the gate terminal. For arelatively high power voltage of VDD1=2.0 V, there is little variationof drain current caused by varying the threshold voltage as shown by thedistance between points A1 and B1, whereas for a relatively low powervoltage of VDD2=1.0 V, the drain current is varied greatly by thevariation of threshold voltage as shown by the distance between pointsA2 and B2. The distance between these points further increases when thepower voltage is 1 V or lower.

As a variant embodiment derived from the embodiment of FIG. 12, in whichthe circuit blocks LOG10-LOG30 are supplied with a common oscillationoutput CLK10 from one operation-mode dependent oscillation circuitVCLK10, two operation-mode dependent oscillation circuits VCLK20 andVCLK30 may be provided as shown in FIG. 14. The two oscillation circuitsVCLK20 and VCLK30 are supplied with a common system clock SCLK20 andmode signal MODE20.

The circuit blocks LOG10-LOG30 have individual oscillation circuitsOSC10-OSC30 and control circuits CNT10-CNT30, of which the controlcircuits CNT10 and CNT20 are commonly supplied with the oscillationoutput CLK20 of the oscillation circuit VCLK20 and the control circuitCNT30 is supplied with the oscillation output CLK30 of the oscillationcircuit VCLK30.

The operation-mode dependent oscillation circuits VCLK20 and VCLK30,control circuits CNT10-CNT30, and oscillation circuits OSC10-OSC30 arearranged as shown in FIG. 4. The scheme of controlling multiple circuitblocks LOG10 and LOG20 with a single operation-mode dependentoscillation circuit VCLK20 is identical to the embodiment of FIG. 12,and the same effectiveness is attained.

Based on this circuit arrangement, circuit blocks, e.g., LOG10 andLOG30, can be controlled separately with different operation-modedependent oscillation circuits, e.g., VCLK20 and VCLK30, so thatindividual circuit blocks can operate under suitable substrate biascontrol. For example, based on the setting of optimal threshold voltagesfor individual circuit blocks, a circuit block can operate at a higherspeed in exchange for a larger subthreshold leakage current and anothercircuit block can operate at a smaller subthreshold leakage current inexchange for a lower operation speed.

With respect to the unequality of threshold voltage among transistors asmentioned previously in connection with the embodiment of FIG. 12, it isdesirable for this embodiment to divide the main circuit into circuitblocks on a semiconductor chip based on the operation speed so thattransistors located close to each other are allotted to a same circuitblock.

Although in FIG. 14, the operation mode signal MODE20 is fed commonly tothe oscillation circuits VCLK20 and VCLK30, these oscillation circuitsmay be supplied with different operation mode signals so as to producedifferent output frequencies. This modified circuit arrangement enablesa certain circuit block including a main circuit for a specific logicaloperation to operate at a higher or lower speed. For example, if acircuit block LOG30 is a microprocessor having a floating-pointarithmetic unit, only this circuit block can be controlled to operate ata higher speed during the floating-point arithmetic process.

Although in this embodiment, the circuit blocks LOG10 and LOG20 arecontrolled commonly with the oscillation output CLK20 of theoperation-mode dependent oscillation circuit VCLK20, these circuitblocks may be controlled by oscillation outputs of differentoperation-mode dependent oscillation circuits.

Although in the embodiments of FIG. 12 and FIG. 14, the main circuit andits associates are divided into three circuit blocks, they may bedivided into a larger number of circuit blocks, and the above-mentionedeffectiveness becomes more pronounced as the circuit is divided intomuch smaller circuit blocks.

The circuit blocks LOG10-LOG30 may be formed on a single LSI chip, ormay be formed on multiple LSI chips dividedly. In the latter case,substrate bias control circuits (CNT10, etc.) and correspondingoscillation circuits (OSC10, etc.) are formed on individualsemiconductor chips and an operation-mode dependent oscillation circuitis formed commonly for these semiconductor chips, for example. It isdesirable to form oscillation circuits (OSC10, etc.), which function tomonitor the threshold voltage of main circuits, on the samesemiconductor chips of the main circuits so that their thresholdvoltages are controlled accurately.

FIG. 16 shows the arrangement of a microcomputer based on an embodimentof this invention. The microcomputer 500, which is formed on a singlesemiconductor substrate based on the CMOS technology or the like,receives a clock signal 502 from a fixed-frequency crystal oscillator501 and an operation mode signal 503. The operation mode signal 503which consists of one bit or more is produced by a load detector 505within the microcomputer 500. The load detector 505 detects the volumeof processing of the microcomputer 500 and produces the operation modesignal 503 which controls the operation speed of the microcomputer 500to meet the volume of processing.

The load detector 505 may be formed of a semiconductor circuit includedin the microcomputer 500, or may be accomplished in the form of aprogram executed by the microcomputer 500. Alternatively, it may be anexternal device adapted to monitor the load of the microcomputer 500.

If the microcomputer 500 runs at a small load, the load detector 505delivers an operation mode signal of a small value, by which theoscillation circuit VCLK1 is controlled to produce a lower operationclock signal 504 which determines the operation speed and powerconsumption of the microcomputer 500. If the load of the microcomputer500 increases, the load detector 505 produces an operation mode signal503 of a large value, and the microcomputer 500 runs at an increasedoperation clock frequency to meet the load condition.

In this manner, the microcomputer 500 can run at an optimal operationclock frequency that meets the load condition. The operation mode signal503 may be an analog signal instead of a multi-bit digital signal.

FIG. 17 shows a microcomputer system based on an embodiment of thisinvention. The microcomputer system 600 which includes the microcomputershown in FIG. 16 is connected to an input device 601 such as a keyboardor mouse device through a system bus 602 which is controlled by themicrocomputer 500.

A load detector 505 monitors the active time of the input device 601 andproduces an operation mode signal 503. During a period of frequentoperation of the input device 601, the operation mode signal 503specifies such an operation mode that the microcomputer 500 runs at ahigher clock frequency, or during infrequent operation of the inputdevice 601, the microcomputer 500 runs in operation mode of a lowerclock frequency.

Generally, infrequent operation of the input device 601 signifies asmall load of the microcomputer 500. Based on this manner of loadDetection for the microcomputer 500, the power consumption can bereduced without reducing the practical operation speed of themicrocomputer system.

Although the microcomputer system of FIG. 17 uses an input device 601such as a keyboard or mouse device for the estimation of the load ofmicrocomputer, the load condition may be detected in a different manner.For example, the load of the microcomputer may be estimated by measuringthe time length of a user's CPU access. It is also possible for thesystem to allow the computer user to set the operation mode throughinput device 601.

According to the foregoing embodiments of this invention, the thresholdvoltage of MOS transistors of the main circuit LOG1 can be controlled byuse of the operation mode signal MODE1. For example, for higher-speedoperation of the main circuit, an operation mode in which theoperation-mode dependent oscillation circuit VCLK1 produces ahigher-frequency output CLK1 is specified from outside so that thethreshold voltage of MOS transistors of the main circuit LOG1 islowered. In this case, the main circuit LOG1 operates fast in exchangefor an increased power consumption due to an increased subthresholdcurrent. For lower-speed operation of the main circuit, an operationmode in which the operation-mode dependent oscillation circuit VCLK1produces a lower-frequency output CLK1 is specified from outside so thatthe threshold voltage of MOS transistors of the main circuit LOG1 israised. In this case, the main circuit LOG1 operates at a reduced powerconsumption due to a reduced subthreshold current.

What is claimed is:
 1. A microcomputer comprising:a main circuitcomprising MIS transistors formed on a single semiconductor substratewhich implements a certain logical processing; an oscillation circuitcomprising MIS transistors formed on said single semiconductor substratewhich effectively monitors delay characteristics of said main circuitand produces an oscillation output having a frequency that is variable;an operation clock signal supply circuit which receives a mode signaland a clock signal, and produces an operation clock signal having afrequency determined by said mode signal; a control circuit whichreceives said operation clock signal and said oscillation output findproduces a control signal for controlling a threshold voltage of the MIStransistors of said main circuit and a threshold voltage of MIStransistors of said oscillation circuit such that the frequency of saidoscillation output corresponds to the frequency of said operation clocksignal; and a load detecting means which detects a load of said maincircuit and outputs said mode signal, corresponding to an operation modeof said microcomputer, to said operation clock signal supply circuit,whereby the operation mode determines whether the microcomputer shouldrun with higher speed performance at higher power consumption or wishlower speed performance at lower power consumption.
 2. A microcomputeraccording to claim 1, wherein said load detection means is embodied insoftware which is executed by said microcomputer.
 3. A microcomputeraccording to claim 1, wherein said threshold voltage of MIS transistorsis controlled by controlling substrate bias such that said thresholdvoltage becomes a low value when said mode signal corresponds to a highspeed operation with high power consumption or a high value when saidmode signal corresponds to a low speed operation with low powerconsumption.
 4. A microcomputer according to claim 1, wherein saidsubstrate bias is applied in a forward direction to a pn junction formedbetween said semiconductor substrate and a diffusion layer connected toa source electrode of at least one of said MIS transistors.
 5. Amicrocomputer system including a microcomputer according to claim 1, andfurther comprising:an input device and a system bus, said microcomputerbeing connected with said input device via said system bus which iscontrolled by said microcomputer.
 6. A microcomputer comprising:firstand second main circuits comprising MIS transistors which implementcertain logical processings; first and second oscillation circuitscomprising MIS transistors formed on said semiconductor substrate whicheffectively monitor delay characteristics of said first and second maincircuits, respectively, and produce first and second oscillationoutputs, each of the oscillation outputs having a frequency that isvariable; first and second operation clock signal supply circuits whichreceive first and second mode signals, respectively, and a clock signal,and produce first and second operation clock signals having afrequencies determined by said first and second mode signals whichcorrespond to first and second operation modes, respectively; a firstcontrol circuit which receives said first operation clock signal andsaid first oscillation output and produces a first control signal forcontrolling a threshold voltage of the MIS transistors of said firstmain circuit and a threshold voltage of MIS transistors of said firstoscillation circuit such that the frequency of said first oscillationoutput corresponds to the frequency of said first operation clocksignal; a second control circuit which receives said second operationclock signal and said second oscillation output and produces a secondcontrol signal for controlling a threshold voltage of the MIStransistors of said second main circuit and a threshold voltage of MIStransistors of said second oscillation circuit such that the frequencyof second said oscillation output corresponds to the frequency of secondsaid operation clock signal; a first load detecting means which detectsloads of said first main circuit and outputs said first mode signalcorresponding with a first operation mode fed to said first operationclocks signal supply circuit; and a second load detecting means whichdetects loads of said second main circuit and outputs said second modesignal corresponding with a second operation mode fed to said secondoperation clock signal supply circuit, and whereby the first maincircuit of the microcomputer can be controlled at a first operationspeed and power consumption independently from a second operation speedand power consumption of the second main circuit.
 7. A microcomputersystem according to claim 5, wherein said mode signal can be set throughsaid input device operated by a user of said microcomputer system.
 8. Amicrocomputer system according to claim 5, wherein said load detectionmeans detects a rate of operation of said input device.
 9. Amicrocomputer comprising:a main circuit comprising MIS transistorsformed on a semiconductor substrate which implements a certain logicalprocessing; an operation clock signal supply circuit which supplies anoperation clock signal, a frequency of said operation clock signal isset at a first frequency in accordance with a first operation mode ofsaid main circuit or a second frequency lower than said first frequencyin accordance with a second operation mode of said main circuit; anoscillation circuit comprising MIS transistors formed on saidsemiconductor substrate which produces an oscillation output having afrequency that is variable; and a control circuit which receives saidoperation clock signal and said oscillation output and produces acontrol signal for controlling a threshold voltage of the MIStransistors of said logic circuit and said oscillation circuit such thatthe frequency of said oscillation output corresponds to the frequency ofsaid operation clock signal; whereby a power consumption of said maincircuit operating in the first operation mode is larger than the powerconsumption of said main circuit operating in the second operation mode.10. A microcomputer according to claim 9, further comprising:anoperation clock signal generator which receives a clock signal andproduces said operation clock signal having a frequency in accordancewith the operation mode of said main circuit, wherein the operation modeof said main circuit is determined according to a load of said maincircuit.
 11. A microcomputer comprising:a main circuit, comprising MIStransistors formed on a semiconductor substrate, which implements acertain logical processing; a monitoring circuit which produces amonitoring signal reflecting a delay characteristic of said maincircuit; and a control circuit which receives said monitoring signal anda reference signal in accordance with an operation mode of said maincircuit and produces a control signal for controlling a thresholdvoltage of the MIS transistors of said main circuit so that a frequencyor a phase of said monitoring signal corresponds to a frequency or aphase of said reference signal, wherein a first reference signal is usedas said reference signal for a first operation mode of said maincircuit, and a second reference signal is used as said reference signalfor a second operation mode of said main circuit, and whereby a powerconsumption of said main circuit operating in the first operation modeis different from the power consumption of said main circuit operatingin the second operation mode.
 12. A microcomputer according to claim 11,further comprising:a reference signal generator which produces saidreference signal in accordance with the operation mode of said maincircuit; wherein the operation mode of said main circuit is determinedaccording to a load of said main circuit.
 13. A microcomputer whichcontrols power consumption by controlling its operation speedcomprising:a main circuit formed on a single semiconductor substrate andcomprising transistors and performing logical processing; a loaddetector which effectively detects a load of said main circuit andgenerates a mode signal corresponding to said load; an operation clocksignal supply circuit which receives said mode signal and a clocksignal. and generates an operation clock signal of a frequencydetermined by said mode signal; and a control signal which receives saidoperation clock signal and produces a control signal for controlling athreshold voltage of the transistors of said main circuit so that adelay characteristics of the transistors of said main circuit is changedin accordance with a frequency of said operation clock signal; whereinwhen the load of the main circuit as detected by the load detector islow, the mode signal of the load generator is generated to decrease thepower consumption and operation speed of the microcomputer if the powerconsumption and operation speed are too high in view of the detectedload, and wherein when the load of said main circuit is high as detectedby the load detector, the mode signal of the load generator is generatedto increase the power consumption and operating speed of themicrocomputer if the power consumption and operating speed are too lowin view of the detected load.
 14. The microcomputer according to claim13, wherein the power consumption and operating speed of themicrocomputer are controlled by the mode signal by using a controlcircuit which receives the operation clock signal and controls athreshold voltage of the transistors of the main circuit.
 15. Themicrocomputer according to claim 13, wherein the load detector is formedfrom a semiconductor circuit.
 16. The microcomputer according to claim13, wherein the load detector is implemented by a program executed bythe microcomputer.
 17. The microcomputer according to claim 13, whereinthe mode signal is a digital signal.
 18. The microcomputer according toclaim 13, wherein at least a portion of the load detected by the loaddetector is based upon a frequency of operation of an input device thatis external to the microcomputer.
 19. The microcomputer according toclaim 13, wherein at least a portion of the load detected by the loaddetector is based upon an amount of time a user accesses themicrocomputer.
 20. The microcomputer according to claim 13, wherein themode signal may be set by a user independently of the mode signalgenerated by the load detector in response to the load detected by theload detector.
 21. A microcomputer which controls power consumption bycontrolling its operation speed comprising:a main circuit comprising MIStransistors formed on a semiconductor substrate which performs logicalprocessing; a load detector which effectively detects a load of saidmain circuit and generates a mode signal for controlling at least one ofoperation speed and power consumption of said microcomputer; anoperation clock signal supply circuit which receives said mode signaland a clock signal and generates an operation clock signal of afrequency determined by said mode signal; and a control circuit whichcontrols a threshold voltage of said MIS transistors; wherein when saidload detector generates a first mode signal as said mode signal, saidthreshold voltage of said MIS transistors is a first value, and whensaid load detector generates a second mode signal as said mode signal,said threshold voltage of said MIS transistors is a second value lowerthan said first value.
 22. A microcomputer according to claim 21,wherein said operation speed or power consumption of said microcomputerwhen said load detector generates said first mode signal is lower thanthat when said load detector generates said second mode signal.